1. Field of the Invention
The invention relates to a frequency selective radome and in particular to a structurally rigid symmetric, electrically conductive substrate structure having symmetric, two pole, iris and loaded circular waveguide elements.
2. Description of the Prior Art
Frequency selective radomes for aircraft constructed from conventional printed circuit RF filter elements, sometimes referred to as frequency selective surfaces (FSS) are known. Fragmentary views of a typical thin screen radome 10 constructed according to the prior art is illustrated in FIGS. 1A-1C. In FIG. 1A some layers have been removed for clarity. In the radome 10, dipole type RF circuit elements 12 are etched on copper foil sheets 14 which are supported into opposite sides of a dielectric substrate 16. The elements 12 include aperture portion 18 and conductive patches 20 which establish a resonant circuit in the space separating the conductive patches. One or more matching dielectric layers 24, 26 are disposed on opposite sides of the device atop the copper foil layers 14 as illustrated.
The arrangement in FIG. 1A is constructed entirely from sheets of dielectric laminated with the metal foil layers 14 and can result in designs with undesirable structural and electrical characteristics. For example, mechanically, the dielectric layers can be relatively thin, are fairly brittle and offer little structural strength. It is difficult to terminate or feather the marginal edge of the dielectric into the mechanical skin of the aircraft. Electrically, the dielectric structure can trap surface waves occurring in the dielectric 16 which results in poor scattering properties. It also can be difficult to scan compensate such devices.
So called "puck" plates which are illustrated in FIGS. 2A-2B are single pole devices employing a relatively thick conductive substrate 32 having circular apertures 34 therein. Rigid ceramic high dielectric constant discs 36 are located in the apertures. The process for manufacturing such devices is extraordinarily time consuming and expensive because the ceramic discs 36 are individually located by hand into in each of the corresponding holes 34. The discs must be installed by hand because a high dielectric constant ceramic is required which cannot be made pourable. One or more dielectric matching layers 38, 40 may be provided as illustrated.
The arrangement illustrated in FIGS. 2A-2B is a single pole device which has relatively poor frequency selectivity. Accordingly, in order to achieve the higher selectivity of a two pole device, a pair of apertured plates 32 are stacked with an interlayer of dielectric material 42 therebetween (FIG. 2C). The devices illustrated in FIGS. 2A-2C are structurally more sound than the thin screen dielectric devices 10 (FIGS. 1A-1C), but are difficult to manufacture and may also be difficult to scan compensate. They also may trap surface waves in the dielectric layer 42.
Devices are fabricated taking into account the desired bandwidth, frequency selectivity, and frequency roll off characteristics. Mechanical parameters including overall geometry such as aperture size and shape, and the electrical properties such as dielectric constants and conductivity of the various layers are all interdependent properties which effect the performance of the final design. These properties must be carefully chosen so that desired performance is achieved.